1. Field of the Invention
This invention relates to removal of organic materials from water streams. More particularly, this invention relates to a reactor for efficient oxidation or ionization of various organic materials commonly present in water streams, to be used in combination with apparatus for removal of the ionized organic materials, and to an accompanying method for oxidation, ionization, and removal of such organic materials.
2. Description of the Prior Art
For many years, there has been substantial interest in removal of organic materials from water streams. "Removal", in this context, includes destruction of organic molecules, e.g., by oxidation to form weak organic acids and carbon dioxide, as well as removal of the organics per se. This subject includes removal of bacteria and other organic wastes from various sources in effluent streams, to render the effluent potable or at least non-polluting, removal of organic solvents and other industrial chemicals from a process water stream for reuse or for discharge, and removal of organics from makeup water streams in power plants and the like, as well as destruction of organics in instruments as a step in their measurement.
More recently, traditional methods of destruction of organics in water streams, such as biological digestion, as commonly practiced by sewage treatment plants, have been supplemented by photocatalytic destruction of organics. In the latter processes, the water stream containing the organics to be destroyed is exposed to ultraviolet radiation (UV) of wavelength suitable to break up the organics, typically UV of 185 and 254 nm wavelength, in the presence of a semiconductive catalyst, commonly a TiO.sub.2 layer on the surface of a solid substrate.
The theoretical basis for photocatalysis in general is discussed in Bard, "Photoelectrochemistry and Heterogeneous Photocatalysis at Semiconductors," J. Photochemistry 10, 59-75 (1979), in Arakawa, "The Present Status and Trends of Photocatalytic Reactions," TechnoJapan 18, No. 11, 10-22 (1985), and is summarized in Fox, "Organic Heterogeneous Photocatalysis: Chemical Conversion Sensitized by Irradiated Semiconductors," Acc. Chem. Res. 16, 314-321 (1983).
Commonly-assigned U.S. Pat. No. 4,868,127 to Blades et al (incorporated herein by this reference) discloses and claims an instrument for measurement of total organic carbon (TOC) in water, wherein a TiO.sub.2 layer formed on the surfaces of solid Ti electrodes used to monitor the conductivity of a water sample catalyzes the oxidation of organics in the water sample upon exposure to UV. Blades et al suggest at col. 24 that the combination of short-wavelength UV and a photocatalyst such as TiO.sub.2 may be useful in the destruction of organics in water, specifically for pollution control purposes. Blades et al suggest further at col. 25, line 34 col. 26, line 10 that application of a potential across electrodes exposed to the water, typically 150 V, is useful in speeding the reaction of the organics to form CO.sub.2.
Pool, "Sun-Powered Pollution Clean Up," Science 245, 130 (Jul. 14, 1989), discloses an experimental-scale device in which polluted water is mixed with grains of TiO.sub.2 and pumped along the bore of a glass tube at the focus of a parabolic reflector collecting the sun's rays. According to this disclosure, UV from the sun frees electrons from the TiO.sub.2, creating electron "holes". The holes combine with water, dissolved oxygen, and naturally-present hydrogen peroxide to create hydroxyl radicals and peroxide ions. The latter break the organics into water, CO.sub.2, and easily-neutralized dilute acids. Pool suggests that this basic approach may be useful in removing ordinary organic wastes as well as common organic solvents and the like, such as trichloroethylene. Pool does not address the problem of removal or recovery of the grains of TiO.sub.2 used to catalyze the reaction.
A number of recent patent documents disclose various features of systems for photocatalytic destruction of organics in water. For example, U.S. Pat. No. 4,888,101 to Cooper discloses disposition of semiconductor powders in a fiber mesh consisting of glass wool or the like. U.S. Pat. No. 5,174,877 to Cooper et al teaches mixing such a powder with water to form a slurry and continuously stirring the slurry to maintain its consistency. U.S. Pat. No. 5,116,582 to Cooper et al discloses a photocatalytic reactor featuring a wire wrapped around a UV lamp to ensure turbulence in the water stream. A potential of several volts between the wire and the water stream is disclosed to be of value in increasing the rate of decomposition of the organics. Another Cooper et al U.S. Pat. No. 5,118,422, is directed to certain filtration steps intended to separate the semiconductor powder from the water stream and to back-flushing the filter from time to time. It is essential to remove the highly abrasive TiO.sub.2 particles from the water. Particles in general are highly undesirable in a water stream, particularly in semiconductor processing.
European patent application 0 234 875 to Kawai et al discloses photocatalytic removal of trace organics from ultrapure water, e.g., as commonly used in semiconductor manufacture, and suggests that the optimum catalyst consists of "anatase" TiO.sub.2 (that is, TiO.sub.2 in the "octohedrite" crystal structure, rather than the more common "rutlie" form) in particulate form, with the particles optimally between about 100 m.mu. and 10 .mu. in average diameter. Addition of Pt is also taught to increase the catalytic activity. See page 14, lines 9-10.
U.S. Pat. No. 4,892,712 to Robertson et al discloses purification of water or air using a reactor comprising a length of transparent mesh coated with a photocatalytic material.
U.S. Pat. No. 4,861,484 to Lichtin et al teaches the use of a "solid catalyst comprising at least one transition element" in combination with an oxidizer, typically hydrogen peroxide, for photocatalytic decomposition of various organic substances. The "solid" catalyst appears to be prepared in particulate form and mixed with the water to be purified to form a slurry.
The inventors are also aware that a number of disclosures (not forming part of the prior art applicable to this invention, but discussed here for completeness) have been made orally, notably at a November, 1992 conference on "TiO.sub.2 Photocatalytic Purification and Treatment of Water and Air" held in London, Ontario, Canada. Written Proceedings of this conference are not available as of the filing date of this application. The inventors are advised that numerous experiments involving immobilized catalysts exposed to UV to catalyze destruction of organics were discussed. Typically, a TiO.sub.2 coating is formed on a fiber mesh (as in the Robertson patent), on a fixed bed, on a film, or as a membrane. TiO.sub.2 films formed on Ti metal, as in the electrodes shown in the Blades et al patent, were also discussed.